A Treatise on Painting, by Leonardo da Vinci
Senex and Taylor, London
Observe, that where your Shadows terminate, there be always an Appearance of a half Shadow; that is, a mixture of Light and Shadow; and that the Shadow be more perfectly mingled with the Light, as it is further removed from the dark Body which projects it. Now that Colour of a Body is never seen simple; this I have already proved, on this Principle; that the Surface of every Body partakes of the Colour of its Object; even though it be the Surface of a Transparent Body, as Water, Air, or the like: For Air borrows Light from the Sun, and darkness from the upper Regions; and further, it is seen tinged with as many different Dies, as there are are different Colours between which, and the Eye it is interposed. Now though the Air, like Water, and other transparent Bodies, has no Colour of its own; yet the moist Vapours emitted from the Earth, and received into the lower Regions of the Atmosphere, thickening and constipating that part of the Air near the Earth; the Sun-beams in their Passage through it, leave Part of their Light; being unable to make their Way through so gross a Medium, without being reflected every Way: so that the upper Regions of the Atmosphere, at the same time, remaining Dark; the Air, hence becomes tinged with Azure; that being the Result of Light and Darkness mingled together; and the Brightness or Obscurity of this Azure will be found to vary, as the Air is more or less charged with these Humid Exhalations.